Spain has confirmed it is considering the introduction of fees for crossing its border with Gibraltar Photograph: Marcos Moreno/AP

Britain intensified its diplomatic protests over Spain's threats to the Gibraltarian border on Tuesday when the British ambassador in Madrid issued a formal demarche, or complaint.

He was told that Spain has not yet decided to impose border fees for anyone crossing the Gibraltar-Spanish border, but the Spanish minister for foreign affairs, Gonzalo de Benito Secades, confirmed the government had the proposal under active legal consideration, as well as a series of other measures.

The Spanish also told the ambassador, Giles Paxman – who is the brother of the BBC broadcaster Jeremy – that the Gibraltarians should remove the 70 concrete blocks dumped in the bay saying the action breached international maritime law. Gibraltarians protested that the blocks had been dropped there after consultation with the Spanish.

Downing Street must now decide whether David Cameron should on Wednesday ring the Spanish prime minister, Mariano Rajoy, to protest – which would raise the diplomatic temperature further.

The current Brtish judgement is that the issue is unlikely to be resolved quickly, and the Spanish have a political interest in keeping the row simmering.

It is also being considered whether either the Gibralterians or the government in London should lodge a formal complaint with the European Commission over the threat of 50 euro border fees, as well as the delays on the border.

A legal action might take a long time to resolve but in the short term it might force Rajoy to promise not to impose the fees for the foreseeable future.

The Spanish have claimed the delays are justified by the need for police to stop cars and check for smuggling.